Smart munitions have improved the effectiveness of military operations. The high precision and reliability of these munitions have optimized kill ratios and have minimized collateral damage to noncombatants. An example of the communications system enabling smart munitions is shown in U.S. Pat. No. 6,020,854 to Jagnow, et al., the disclosure of which is incorporated by reference herein in its entirety. According to Jagnow, a fuse onboard an artillery shell includes a monopole antenna disposed along the axis of rotation of the artillery shell. The antenna is configured to receive location signals, such as global positioning system (GPS) signals, from orbiting satellites or other sources. Control means, such as retractable flaps, are controlled by circuitry within the artillery shell to adjust the trajectory of the shell during flight.
Because of the effectiveness of smart munitions using GPS for range correction, it is expected that adversaries will try to disrupt L-band GPS signals using ground-based, sea-based or airborne jammers. Effective GPS signal jamming denies the artillery shell the information necessary for precision operations. Therefore, it is important to know whether an adversary is attempting to jam the signal. It would also be very desirable to know the location of a jammer by locating its counter attack or counter measure (CM) radar (e.g. C or S-band radar) so the jammer and/or radar system could be monitored or neutralized. A radio direction finding (RDF) system aboard the munition could be used to detect CM radar signals and locate the source thereof. This would provide the munition with counter counter measure (CCM) capability and mitigate jamming in target approach. An RDF system to detect a jammer's CM radar system that could be operated without risk to friendly personnel is greatly needed. Also needed is an RDF system that can be used to detect and locate the sources of other types of communications in the C and/or S bands, such as satellite communication data links, information friend-or-foe (IFF) systems, or the like.
One type of RDF technology takes advantage of a well-known principle called the Doppler effect. A change in frequency, known as a Doppler shift, occurs in a radio signal when a signal source and sensor move with respect to each other. The shift is proportional to the relative speed difference between them. In a Doppler RDF system a rotating sensor experiences a predictable phase shift from an approaching signal as the sensor moves along a circular path. In accordance with the Doppler effect, the frequency of the carrier signal increases as the sensor moves along its circular path toward the carrier signal source, and the frequency decreases as the sensor moves away from carrier signal source. Thus, the source's carrier signal is frequency modulated with the rotation of the sensor. By differentiating the instantaneous amplitude of the carrier and filtering out the DC carrier frequency component, a demodulated Doppler signal is obtained. The demodulated Doppler signal phase is referenced to the antenna rotation frequency to determine the bearing.
Practically, to obtain an easily measured Doppler shift, a high rotational sensor speed is required. Due to the practical limitations of rotating an antenna at high speed, many RDF antenna systems use a fixed array of antennas and a rotational mechanical switch, or an RF electrical switch circuit to commutate the circular array of antennas to a central feed point. Such solutions may be impractical when rotating at extremely high rates or launched at extremely high accelerations, as is typically required of an artillery shell during launch and flight. Furthermore, these solutions are not optimal for the small size constraints of an artillery fuse, and may be too cost-prohibitive to be used in an artillery fuse that by design is intended to be used only once. When physical size, weight and power are design constraints, or a rugged, efficient low cost RDF system is needed, such as the requirements for antenna systems installed within an artillery fuse, another approach is needed.
It is therefore an object of the invention to provide an RDF system that can be used to detect a GPS jammer's CM radar system without endangering friendly personnel, thus providing CCM capability onboard smart munitions.
It is another object of the invention to provide an RDF system that can be used to detect and locate other types of communications or radars in the C and/or S bands.
It is another object of the invention to provide an RDF system that can be used in an environment having extreme spin or roll rates.
It is another object of the invention to provide an RDF system that can be embedded in an artillery fuse.
A feature of the invention is a monopole antenna having a radiator disposed parallel but separate from the axis of rotation of a spinning object.
An advantage of the invention is an RDF system that can survive extreme accelerations and high roll rates, such as those found in an artillery fuse.
Another advantage is that the RDF system is inexpensive and easy to manufacture.
Still another advantage is that the RDF system can be tailored to function at various frequencies in the electromagnetic spectrum.